The present invention relates to a multi-stage counter-current crystallization purification process for use in chemical operations where a high degree of purity is required for the end product.
A number of different crystallization systems have been proposed to purify various chemical feeds or to recover a specific component from a feed mixture, but none of the systems employed presently have been completely satisfactory. The disadvantages with the present systems are that they are either unable to achieve the required level of purity, or else they are extremely complicated and expensive in both capital equipment and operating costs. Accordingly, crystallization purification has had only limited application in the chemical process industry.
Crystallization purification is also employed in the food process industry. One system which has been successful is the two stage continuous process concentrating system described in the patent to Thijssen et al, U.S. Pat. No. 4,004,886. In this system a slurry of seed ice crystals and mother liquor are produced in a scraped surface heat exchanger and continuously supplied to a recrystallizer in which the crystals grow. The mother liquor in the recrystallizer is continuously mixed and recirculated to the scraped surface heat exchanger via a filter which prevents crystals from leaving the recrystallizer. The mother liquor is consequently mixed with the return of the wash column and the process feed, passes the scraped surface heat exchanger and is recirculated to the recrystallizer. Almost all of the small crystals in the recrystallizer melt and reform on a few seed crystals to produce a crystal slurry having relatively uniform-sized crystals therein, which slurry is removed continuously from the recrystallizer as a crystal suspension. While others in the past have proposed systems using both scraped surface heat exchangers and larger tanks in which crystal growth takes place, for example, the patent to Walker U.S. Pat. No. 3,156,571, it is the melting of the vast majority of the ice crystals provoked by the recirculation only of substantially crystal free liquid from the recrystallizer which reduces the residence time and produces the uniform crystal size in both the system described in the Thijssen et al patent. The uniform crystal size in particular permits use of wash columns in the system instead of centrifuges or other separating devices which have technical and other disadvantages.
The slurry which is removed from the recrystallizer in the above-described system of the Thijssen et al patent is preferably supplied to a wash column, for example, as described in the Thijssen U.S. Pat. Nos. 3,872,009 and 3,777,892. In this particular wash column, the slurry is supplied to the bottom of a column and then compacted against the ice mass by a piston which periodically pushes the mass upward. The ice at the top of the column is chopped and removed from the column. Subsequently, it is melted and at least in part returned to the column to flow downward when the piston applies pressure to the bottom of the column to maintain a stable wash front. The mother liquor is removed as concentrated liquor through perforations in the piston. Other subsequent systems operate continuously.
Subsequent work resulted in the development of a multi-stage system using the method and apparatus of the Thijssen '886 patent as the basic component. A plurality of Thijssen et al type concentrating units are connected together for counter-current operation.
In a first stage which receives the feed liquid to be concentrated and produces a first intermediate concentration solution, a slurry of ice crystals and liquid in a recrystallization vessel are supplied to a separator such as a wash column and the intermediate concentrated solution is passed to a second stage. The ice crystals from the second stage are passed counter-current to the direction of flow of the aqueous liquid and supplied to the recrystallization vessel of the first stage. Almost all of the seed crystals produced in the first stage, for example, by a scraped surface heat exchanger, then melt and reform upon the larger crystals supplied from the second stage. Third and additional stages can also be provided, each passing at least the ice back directly to the recrystallization vessel of the preceding stage so that the crystal growth takes place mainly on the crystals which are supplied from the succeeding stage and practically all of the crystals generated in each stage except the last melt and reform thereon.
This multi-stage system is described in, e.g., Van Pelt et al, U.S. Pat. Nos. 4,316,368, 4,430,104 and 4,459,144.
It has now been discovered that a multi-stage counter-current crystallization system can be adapted for use in the chemical process industry for recovery and purification of chemicals.
The present invention makes use of substantially identical stages in series. The number of stages is primarily dependent on the specific requirements of the feed mixture, the final product, and the waste product. In one embodiment the unit process is comprised of a recrystallizer with a filter, a crystallizer, a separator; either a thickener or wash column, and associated piping and pumping equipment. This basic unit is then simply repeated sequentially as many times as is necessary for the desired level of purification and/or recovery of a specific chemical in the feed mixture. This concept reduces both engineering time and cost as well as equipment manufacturing time and cost. The unit process is divided into two sections, a recovery section and a purification section. Each of these sections has predetermined numbers of sequential stages depending on the specific application. Because the stages are substantially identical, even in the two sections, the unit process of the present invention has remarkable flexibility. The unit process of the present invention is also very energy efficient and is ideally suited for the use of multiple heat pumps.
The processing unit is divided into two sections, a purification section and a recovery section. The feed mixture is fed into a wash column located in the purification section. In certain instances, e.g. when the feed already contains crystals, it can be processed in only that wash column. Some feed mixtures may require a purification section comprised of two or more wash columns in series. If necessary, the purification section can be expanded in stages. Each subsequent stage of the purification section is substantially identical and is comprised of an additional wash column, and possibly but necessarily, also comprised of the following equipment; a recrystallizer intermediate the wash columns, at least one filter unit associated with the recrystallizer, and at least one crystallizer which forms crystals to be fed to the recrystallizer.
The recovery section in one embodiment is comprised of plurality of substantially identical stages. The only stage that is different is the stage adjacent to the first wash column in the purification section. The stage adjacent the wash column is comprised of a recrystallizer, at least one filter associated with said recrystallizer, and at least one crystallizer for forming crystals to be fed to the recrystallizer. No thickener is required because of the presence of the adjacent wash column.
The other stages in the recovery section have a thickener for concentrating the crystals removed from the recrystallizer which are then fed to the recrystallizer in the adjacent stage.